The CDC’s early estimates fulfill the predictions by public health experts who said this season’s re-emergence of the H1N1 flu strain -- responsible for a 2009 pandemic -- would target younger patients the same way it did several years ago.

Patients between the ages of 18 and 64 years old represented more than 60 percent of all hospitalizations from influenza. This is more than the previous three flu seasons, the CDC said, when the age group composed roughly 35 percent of hospitalized patients.

Plus, patients who were 25 years to 64 years old have accounted for about 60 percent of flu deaths this season, the CDC said. That’s compared to 18 percent, 30 percent and 47 percent for the three previous seasons.

"Younger people may feel that influenza is not a threat to them, but this season underscores that flu can be a serious disease for anyone," said Dr. Tom Frieden, the CDC’s director, in a statement.

The government agency’s findings were published in its Morbidity and Mortality Weekly Report and addressed today in a conference call with federal health officials.

The H1N1 virus accounted for 98% of influenza viruses detected, the weekly report said. The H1N1 subtype has sickened patients since its emergence, but the CDC said this season was the first time the virus has been predominant in the United States since the pandemic.

Chicago’s public health department reported 154 flu-related intensive care unit hospitalizations between the end of September 2013 and early February – and 43 percent of those patients were younger than 50 years old, according to city flu reports. At least 46 percent of those ICU patients tested positive for H1N1.

During the 2009-10 flu season, when H1N1 swept across the Chicago area, 72 percent of flu-related intensive-care unit admissions were for patients age 49 or younger. Nearly a third of all such admissions involved patients no older than 18.

By contrast, according to city data, 75 percent of last flu season’s ICU patients were age 50 or older.

Flu activity in Illinois is now on a downward trend, by most measures. But influenza was still classified as “widespread” by the state’s department of public health through Feb. 8. Regional health departments are expected to update flu surveillance information on Friday.

National flu data suggests the virus will continue to circulate for “a number of weeks, especially in places where activity started later in the season,” the CDC said.

“Some states that saw earlier increases in flu activity are now seeing decreases. Other states are still seeing high levels of flu activity or continued increases in activity,” the agency said.

Most injectable flu vaccines this season were meant to protect against three viruses expected to be most common. That includes the H1N1 strain, known as swine flu, one strain of influenza B and the H3N2 virus, which contributed to dozens of intensive care admissions in Chicago last season.